Touch-target audit: 44px minimum on mobile interactive elements (Slice 9)

Apply the mobile-touch-target CSS class to 40 interactive elements across
12 files. The class applies min-height/min-width:44px only below md
(max-width:767px), satisfying WCAG 2.5.5 / Apple HIG on touch devices.
Desktop behavior is unchanged.

Audit log (before -> after hit-area):
- App.tsx: hamburger/dark-mode/sign-out (32/32/28 -> 44)
- Dashboard.tsx: shortcut open/edit/delete (28 -> 44), enabled switch (18 -> 44)
- Media.tsx: mobile pagination prev/next (28 -> 44)
- FileBrowser.impl.tsx: 'Open Settings' alert button (28 -> 44)
- UsersPage.impl.tsx: compose toolbar bold/italic/link/list (32 -> 44),
  attachment remove button (16 -> 44)
- Settings.tsx: machine switch (18 -> 44), clear/add-machine buttons (28 -> 44),
  reset-db checkboxes x3 (16 -> 44)
- Actions.tsx: 'Add action' button (28 -> 44)
- ServicePage.tsx: service enabled switch (18 -> 44)
- ServicesPage.tsx: service switch/open-link/delete-icon (18/28/32 -> 44)
- ObservabilityPage.tsx: retry + 4 asChild link buttons (28 -> 44)
- WidgetConfigDialog.tsx: 4 icon buttons (32 -> 44), 2 switches (18 -> 44),
  2 add-widget buttons (28 -> 44)
- SessionActivityPanel.tsx: 'Open in Users' button (28 -> 44)

Deliberately skipped: default-size text buttons (32px, borderline), desktop-only
sidebar toggle, DataTable internals (desktop-only below md), Select triggers.
Dashboard anchor pills and HoverEditButton already had the class from Slices 1/2.

No new tests (the class applies via @media which jsdom doesn't honor).
116 tests pass; lint/build green.

Refs openspec/changes/mobile-responsive-parity/ (spec R6, tasks slice 9).

frontend/src/App.tsx

@@ -194,7 +194,7 @@ function MobileDrawer() {
 	return (
 		<Sheet open={open} onOpenChange={setOpen}>
 			<SheetTrigger asChild>
-				<Button variant="ghost" size="icon" className="md:hidden">
+				<Button variant="ghost" size="icon" className="mobile-touch-target md:hidden">
 					<Menu className="h-5 w-5" />
 				</Button>
 			</SheetTrigger>
@@ -281,7 +281,7 @@ function TopBar({
 					variant="ghost"
 					size="icon"
 					onClick={onToggleDarkMode}
-					className="h-8 w-8"
+					className="mobile-touch-target h-8 w-8"
 				>
 					{darkMode ? (
 						<Sun className="h-4 w-4" />
@@ -294,7 +294,7 @@ function TopBar({
 						variant="ghost"
 						size="sm"
 						onClick={onSignOut}
-						className="gap-2"
+						className="mobile-touch-target gap-2"
 					>
 						<LogOut className="h-4 w-4" />
 						<span className="hidden sm:inline">Logout</span>